1. Field of the Invention
The present invention relates to an apparatus for detecting the purification factor of a catalyst of a catalytic converter used in an internal combustion engine. The quantity "purification factor", as used herein is a measure of the effectiveness of the catalyst in removing pollutants from the exhaust gas of the engine.
2. Description of the Related Art
Various types of apparatus for detecting the purification factor of a catalyst have been proposed in the prior art (where the term "detecting" as used herein can signify an operation for judging whether the purification factor is above a predetermined reference value, or an operation for actually measuring the purification factor). For example, an apparatus has been proposed which has a pair of O.sub.2 sensors disposed in the exhaust system of an internal combustion engine, respectively upstream and downstream from the catalytic converter, with the purification factor of the catalyst in that catalytic converter being detected based on a response delay of these O.sub.2 sensors. Such a system is described for example in Japanese Patent Laid-open No. 51-55818.
An apparatus is also known in the prior art (for example as described in Japanese Patent Laid-open No. 49-109721) in which a decision is made as to whether or not deterioration of the catalyst has occurred, based on an output value produced from an O.sub.2 sensor that is disposed downstream from the catalytic converter, or based on a relationship between output values that are produced from two O.sub.2 sensors that are disposed respectively upstream and downstream from the catalytic converter.
In controlling the air-fuel ratio of an internal combustion engine, in order to bring the actual air-fuel ratio close to a stoichiometric value of air-fuel ratio, a method is now widely utilized whereby the duration of the fuel injection time intervals is changed by negative feedback control in accordance with output values produced from an O.sub.2 sensor which is disposed in the exhaust system. Specifically, a value for the injection interval duration, which determines a fuel injection quantity, is computed based on the rate of air intake and the engine speed, and that fuel injection quantity is corrected by an air/fuel ratio feedback correction coefficient which is derived based on an output signal produced from the O.sub.2 sensor. Thus, negative feedback control of the fuel injection time interval duration is achieved. In the following, the term air-fuel ratio feedback correction coefficient will be abbreviated to F/B correction coefficient.
Basically, when the O.sub.2 sensor output indicates a lean condition of the air-fuel ratio, the feedback correction coefficient is changed in a direction tending to increase the fuel injection interval duration, whereas when the O.sub.2 sensor output indicates a rich condition of the air-fuel ratio, the F/B correction coefficient is changed in a direction tending to decrease the fuel injection interval duration. As a result of that feedback control, as viewed from the micro aspect, the actual air-fuel ratio varies periodically about a central value which is the stoichiometric air-fuel ratio, while as viewed from the macro aspect, the actual air-fuel ratio is brought close to the stoichiometric air-fuel ratio. The frequency of that variation, i.e. the frequency of variation of the F/B correction coefficient (which is basically identical to the frequency of variation of the output signal from the O.sub.2 sensor) will be referred to as the feedback frequency, and the corresponding period as the feedback period.
As shown in FIG. 2, in the case of a normal value of frequency (for example, 1.5 Hz) of the output signal from the O.sub.2 sensor that is disposed upstream from the catalytic converter in the exhaust system, and is designated as the "front" O.sub.2 sensor, the waveform of the output from the O.sub.2 sensor that is located downstream from the catalytic converter, designated as the "rear" O.sub.2 sensor, successively changes as shown in diagrams (a), (b), (c) and (d) of FIG. 2, in accordance with lowering of the purification factor, as the catalyst deteriorates. That is to say, when the catalyst is new (e.g. the purification factor is 98%) then as shown in diagram (a), the output signal from the rear O.sub.2 sensor varies slowly in amplitude between a high level which corresponds to a rich value of air-fuel ratio and will be referred to in the following simply as the "rich condition" of that signal, and a low level which corresponds to a lean value of air-fuel ratio and will be referred to in the following simply as the "lean condition" of that signal. The signal exhibits large-amplitude changes whose timings are unrelated to the frequency of the output signal from the front O.sub.2 sensor, i.e. are unrelated to the feedback frequency. When the catalyst has slightly deteriorated (e.g. the purification factor is 96%) then as shown in diagram (b), the amplitude of the output signal from the rear O.sub.2 sensor becomes small, however the changes between the rich and lean conditions of the output signal from the rear O.sub.2 sensor are still unrelated to the frequency of the output signal from the front O.sub.2 sensor. As deterioration of the catalyst proceeds (e.g. when the purification factor falls to 80%) then as shown in diagram (c), the output signal from the rear O.sub.2 sensor still varies overall with a long period of variation, but with the high-frequency waveform of the output signal from the front O.sub.2 sensor superimposed upon that variation. When the deterioration of the catalyst reaches a stage such that the purification factor falls to 50% then as shown in diagram (d) the outputs from the rear O.sub.2 sensor and front O.sub.2 sensor are substantially identical in waveform.
Judgment of deterioration of the catalyst based on the output signal waveforms from the upstream and downstream O.sub.2 sensors will now be considered. As shown in FIG. 3, if the frequency of changeover between the rich and lean conditions of the output signal from the front O.sub.2 sensor is relatively long, e.g. 0.7 Hz, then the waveforms of the outputs from the front and rear O.sub.2 sensors will become almost identical, even if substantial catalyst deterioration has occurred. In the prior art, detection of catalyst deterioration has been based on the ratio of the frequencies of the outputs from the front O.sub.2 sensor and rear O.sub.2 sensor, or the ratio of the amplitudes of these outputs. However in many cases, throughout the operating life of the catalyst, the frequency of the output signal from the front O.sub.2 sensor gradually decreases from an initially relatively high value (e.g. 1.5 Hz). If that frequency becomes substantially low (e.g. reaches approximately 0.7 Hz), then it can be understood from FIG. 3 that it will become impossible to discriminate between purification factor values of, for example, 80% and 50%, since the waveforms of the output signals from the front and rear O.sub.2 sensors will be substantially identical, irrespective of the degree of catalyst deterioration. That is clear from the contents of the tables in FIG. 4, which show examples of specific values for the frequency ratio r and the amplitude ratio A of the output signals from the front and rear sensors, at feedback frequency values of 0.5 Hz and 1.5 Hz, for the case of 80% and 50% purification factor values.
Thus, it has not been possible to achieve a sufficiently high accuracy of judging the state of deterioration of the catalyst of a catalytic converter of an internal combustion engine, in the prior art.